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Holotype:- P9464-134 (Figures 7, 8a, b, 9) a fragmented but articulated carapace composed of the nuchal, 1st right peripheral, 1st right costal, left and right costals 2 through 8, right peripherals 4, 5, 7 and partial 9, and left peripherals 4 through 8, and partial 9 and 10; an associated posterior lobe of plastron with incomplete xiphiplastra (Figure 11). All from Top Site, Camfield Station, Northern Territory, Bullock Creek Local Fauna, mid-Miocene.
Diagnosis:- A mid-Miocene fossil species of short-necked turtle of the family Chelidae, assigned to the genus Elseya based on the morphology of the axillary buttress suture scars and pelvic scars and overall carapace osteology and scute sulci; distinguished from all currently known fossil and extant species by the following combination of characters: fully formed cervical scute; anterior to posterior length of anterior nuchal more than 5x that of the posterior; contact of V1-P1 with M1 occurs at posterior 2/3rd position; V1 much wider than long and wider than V2; axillary buttress scar longer than peripheral 1-2 suture and 2-3 suture; axillary buttress scar terminus ends in robust finger like projection; iliac suture scar crosses onto both suprapygal and costal 7; projected supragyal considerably wider than long; presence of a broad, deep pit at the antero-medial corner of the iliac scar occurring on both costals 7 and 8. We tentatively refrain from assignment of this new taxon to the subgeneric Elseya level as it most likely represents an undescribed subgenus.
Description:- The holotype specimen consists a fragmented but articulated carapace composed of the nuchal, 1st right peripheral 1, right costal 1, left and right costals 2 through 8, right peripherals 4,5, 7 and partial 9 and left peripherals 4 through 8, and partial 9 and 10. An associated posterior lobe of plastron consists of an associated right and left hypoplastron missing most of both hypoplastral bridges but preserving the abdominal-femoral sulcus. The anterior portions of both xiphiplastra are respectively present and associated with the hypoplastra and each other. The femoral-anal sulcus is preserved on the right xiphiplastral portion, as is a small reduced pubic scar and broken attached blade on the visceral side. The long axis of the pubic scar is less than a third (29 %) of the proximal width of the xiphiplastron. The left xiphiplastral anal lobe is present but detached and connected via wooden splint, preserving the entirety of the ischial suture scar.
On the carapace, all respective scute sulci are strongly preserved. The surface sculpture of consists of irregular small and minute pock-marking throughout, no formed discernible pattern present, except at the posterior end where the suprapygal is crossed by strong longitudinal ridges separated by deep sulci. These are not as clearly observable on the holotype because the sulci are filled with adherent matrix but are clearly visible in the referred suprapygals (Figure 10). A fully formed cervical scute is present, well-delineated and unlike the infrequent aberrancy of cervical scutes seen in some modern Australian Elseya of the subgenus Elseya. Marginal 1 is wider than long. V1-Pl sulcus is semi-bowed convex exteriorly such that V1 is considerably wider than the prospective width of V2. Regarding the nuchal, the anterior portion of the nuchal is more than 5x that of the posterior portion (character N). The anterior margin of the nuchal is convex in the holotype, whereas it is concave in NTM P9464-136.
Viscerally the suture of dorsal rib 1 and the entirety of the costal axillary suture scar are fully preserved. The axillary scar is a relatively long, overall narrowed but robust structure that retains a consistent width along its full length, showing no medial constrictions. The angle of declination from that of the midline of the carapace on the anterior portion of the scar is nearly 50-55 degrees, rather typical of both extant subgeneric Elseya and ‘Pelocomostes’ taxa. The terminus of the axillary scar lies somewhat distal to the terminus of dorsal rib 1 (character G; Figure 8a, b), the spatial length being short of the peripheral 1-costal 1 suture length (character S) such that the axillary terminus lies directly below the mid-point of the peripheral 1 and costal 1 suture (character H). The terminus of the axillary scar lies posterior of the dorsal rib 1 terminus (character M). The spatial distance between the termina of the dorsal rib 1 and the axillary scar is 1.5x as long as the suture of peripheral 2-costal 1 embedded in the scar (character T, U). The embedded suture is shorter than to the exposed anterior suture of peripheral 2 (character I) and there is no exposed posterior suture (Character I’). The axillary scar base can be projected to be larger in surface area than the axillary terminus (character K); the axillary terminus ends in a robust finger-like projection (character L) and the base of the scar across the costal-peripheral sutures is only slightly wider than the overall width of the terminus (character O). The length of the axillary scar is shorter than the length of the nuchal-peripheral 1-costal 1 conjunction to the center base of the scar (Character P). The axillary scar length is longer than both the lengths of the peripheral 1-2 and 2-3 sutures. The angle of the costal 1-peripheral 1-2 sutures is more than 130 degrees (character V). The overall shape of the axillary scar is an elongated irregular polygon. The base of the axillary scar lies well below the posterior most portion of the nuchal (character J). As the first dorsal vertebra is not preserved, we cannot discern characters X, Y, Z.
Referred Specimens:- NTM P9464-136 (Top Site), nuchal (Figure 8c, d); NTM P9464-590 (Top Site), partial suprapygal (Figure 10c, d); NTM P9973-91 (Top Site) suprapygal (Figure 10a, b).
Etymology:- The species epithet is proposed in honor of the First Peoples of the region, the Mudburra.
Comparison:- Elseya mudburra sp. nov. is assigned to the genus Elseya sensu lato on the basis of the broadly wide axillary suture scar (heavily constricted in extant Emydura and moderately wide in fossil Emydura), by the terminus of the axillary suture scar lying nearly level to slightly posterior of the terminus of dorsal rib one (strongly posterior in fossil and extant Emydura), strongly angular declination of the scar (moderate to acute angular in Chelodina, Pseudemydura; Rheodytes and Elusor); and by the base of the axillary scar lying on the conjunction of the costal 1-peripheral 2-3 sutures (sutures of 3-4 in Chelodina). Elseya mudburra sp. nov. is distinguished from all currently known fossil and extant Australian Elseya species by the following combination of characters: vertebral 1 much broader than 2; presence of a distinct fully-formed cervical scute (absent in extant taxa except as irregular vestigial or aberrant); contact of the V1-P1 sulcus occurring with M1 at the posterior 2/3rd position (excepting E. oneiros and some E. irwini); the anterior portion of the nuchal being more 5x that of the posterior (character N); the shape of the axillary suture scar being an elongated structure (short and blunt in subgenus ‘Pelocomastes’) of equal width along its length (constricted in subgenus Elseya) ending polygonally with robust finger like-projection (spoon-shaped in subgenus Elseya; narrow small finger-like terminus in subgenus ‘Pelocomastes’); the suprapygal being much wider than long (equal to longer than wide in all other known Australian Elseya); the pubic suture scar on the visceral xiphiplastron being a strongly reduced element (large and occupying much more surface area in all other known Australian fossil and extant Elseya); and the presence of a deep broad pit at the antero-medial corner of the iliac scar occurring on both costals 7 and 8, a feature unknown in any other Elseya or Emydura fossil or extant taxa. This pit is a portion of the iliac blade that is short and shallow in extant taxa and confined to costal 8. The large size of the carapace bones clearly separates Elseya mudburra sp. nov. from the dwarf genus Birlimarr. Separated from extant species by some 12 million years.
Elseya mudburra sp. nov. is clearly separated from Elseya camfieldensis sp. nov. by the texture of the surface sculpture on the carapace; the shape, topography, positioning and metrics of the axillary buttress suture scar as all described above; the V1-P1-M1 contact point and the overall shape of V1. Lastly, Elseya mudburra sp. nov. shows a completely interdigitating medial end of the posterior articulating hyoplastral suture, which is uniquely horizontally subdivided into interdigitating and smooth portions in Elseya camfieldensis sp. nov. These species are considered sympatric spatially and temporally in the middle Miocene.
The axillary buttress suture scars of select fossil and extant Elseya and Emydura species are presented in Figure 12.
Comment:- This specimen was referred to as ‘Genus aff. Emydura/Elseya sp. indet.’ by Megirian & Murray (1999).